docs: document release process

[denhamparry]

Adds docs/release.md documenting the Sprout release process, covering:

• Bumping the version in Cargo.toml and verifying the build
• Opening a version bump PR with the correct commit message format
• Creating a signed git tag and pushing it
• Drafting the GitHub release (including the generate-release-notes and blank-line-after-headers steps)
• Dispatching the release workflow with the tag input
• What the workflow does: build, attest, upload, and publish